1. Field of the Invention
This invention relates to the field of tensile strength members such as multi-stranded synthetic cables. More specifically, the invention comprises devices and methods for balancing the load carried by a synthetic cable among its constituent strands.
2. Description of the Related Art
A cable must generally be provided with one or more end connections in order to be useful. The end connections allow the cable to carry and transmit a useful load. An end connection may be a simple device—such as a large hook—employed to connect the cable to an anchoring point. Larger synthetic cables typically include multiple constituent strands. It is preferable to attach an individual connective device to each strand. Such a connective device is referred to in this disclosure as a “strand termination.” Multiple strand terminations are connected together somehow to create a unified cable end connection. The unified cable end connection is referred to in this disclosure as an “overall cable termination.”
For small cables simple end-fittings work fairly well. For larger cables, however, more complicated end-fittings are needed in order to produce acceptable break strength. This is particularly true for large, multi-stranded cables made of synthetic filaments (having diameters of 20 mm or more). FIG. 1 shows a cable 10 made from advanced high-strength synthetic filaments. Some terminology used in the construction of cables will benefit the reader's understanding, though it is important to know that the terminology varies within the industry. For purposes of this patent application, the smallest individual component of the cable is known as a “filament.” A filament is often created by an extrusion process (though others are used). Many filaments are grouped together to create a strand 12. The filaments are braided and/or twisted together using a variety of known techniques in order to create a cohesive strand. There may also be sub-groups of filaments within each strand. As the overall cable size gets larger, more and more layers of filament organization will typically be added. The strands 12 are typically braided and/or twisted together to form cable 10. In other examples the strands may be purely parallel and encased in individual surrounding jackets. In still other examples the strands may be arranged in a “cable lay” pattern that is well known in the fabrication of wire ropes.
Many different materials are used for the filaments in a synthetic cable. These include DYNEEMA, SPECTRA, TECHNORA, TWARON, KEVLAR, VECTRAN, PBO, carbon fiber, nano-tubes, and glass fiber (among many others). In general the individual filaments have a thickness that is less than that of human hair. The filaments are very strong in tension, but they are not very rigid. They also tend to have low surface friction. These facts make such synthetic filaments difficult to handle during the process of adding a termination and difficult to organize. The present invention is particularly applicable to terminations made of such high-strength synthetic filaments, for reasons which will be explained in the descriptive text to follow. While the invention could in theory be applied to older cable technologies—such as wire rope—it likely would offer little advantage and the additional time and expense of implementing the invention would not be worthwhile. Thus, the invention is not really applicable to wire rope and other similar cables made of very stiff elements.
The cable shown in FIG. 1 is a well-known exemplary construction made by braiding or otherwise interrelating twelve strands together. Polyester ropes using this construction are known to have an external diameter up to about 6 inches (see specification MIL-R-24750). Even larger polyester ropes are made by constricting parallel sub-ropes in a braided-strand jacket.
When a cable has non-parallel strands the interrelationship between the strands becomes quite complex. The overall cable has a central axis. Each individual strand is on average running parallel to the cable's central axis. However, at any given point along the cable's length, no individual strand is parallel to the cable's central axis. When such a cable is loaded, the individual strands move and shift. The cable “clinches” together and strand-to-strand friction becomes a significant component of the cable's performance. When a large amount of tension is applied to such a cable in its initial post-manufacturing state, if is known for the cable's diameter to shrink by up to 30%. The individual strands must slip over one another and settle into a stable configuration.
It is important for the overall strength of most cables—the 12-strand configuration of FIG. 1 being a good example—that the overall load be shared equally among the constituent strands. For a 12-strand construction, the ideal result is that each strand carries exactly 1/12 of the total load. Other cables may have a desired non-equal tension distribution, such as a cable having some relatively large strands and other relatively small strands. However, in all cases, it is preferable to have a “target” distribution of tension among the constituent strands and to provide a system that meets this target distribution.
High-strength synthetic filaments have very little surface friction and strands made of these filaments also have very little surface friction. Thus, it is possible for one individual strand to “slip” with respect to neighboring strands. A strand that slips tends to “unload” itself and shift the load it was carrying to its neighbors. This is obviously an undesirable result.
In order to add an overall cable termination to an end of a multi-stranded synthetic cable, each individual strand must be cut to length and have a strand termination added (It is not essential that all strands in the cable undergo this process but in most embodiments all strands will be involved). The cutting and terminating processes are inherently imperfect. The result will generally be that some terminated strands will wind up being longer than desired while others will wind up being shorter then desired. If a tensile load is placed on the cable with no accommodation for these manufacturing tolerances, the relatively “short” strands will be loaded first and they will carry more load than the relatively long strands.
One approach to reducing this problem is to make the application of a tensile load to each strand individually adjustable. In order to achieve this goal a tension-applying apparatus may be applied to each strand termination individually. Looking again at FIG. 1, the reader will note how the strands on the free end of cable 10 have been unbraided so that they are individually accessible.
FIG. 2 shows a section view through a strand termination 30 that has been added to the free end of an individual strand 12. The prior art approaches to adding a termination are explained in detail in commonly-owned U.S. Pat. Nos. 7,237,336, 7,669,294, 8,048,357, 8,236,219, and 8,371,015, which are hereby incorporated by reference. These prior patents generally concern potted terminations, but as discussed previously the invention applies to all types of termination.
FIG. 2 shows a sectional view through the components used to create the termination. The reader will note that anchor 18 includes an expanding cavity 20 that expands as one proceeds from the portion of the anchor facing the length of cable (the “proximal” end, which is the bottom end in the orientation of the view) toward the portion of the anchor facing in the opposite direction (the “distal” end, which is the top end in the orientation of the view). The expanding cavity in this example is a linear taper between two straight portions—all joined by fillets. Differing wall profiles may be used to create a wide variety of expanding cavities.
The end portion of strand 12 is potted into the expanding cavity in order to lock anchor 18 to strand 12. The filaments of the strand are splayed apart and infused with liquid potting compound (either before or after being placed within expanding cavity 22). The liquid potting compound may be added by a variety of methods, including: (1) “painting” or otherwise wetting the filaments with potting compound and then sliding the anchor into position over the painted filaments, (2) positioning the splayed filaments in the cavity and then pouring in potting compound, (3) pre-wetting the filaments in a separate mold designed to wet the filaments, and (4) injecting pressurized potting compound into the cavity. However the potting compound is introduced, the splayed filaments remain within cavity 20 while the potting compound hardens. Once it has hardened the result is a mechanical interlock between the filament-reinforced “plug” (contained in potted region 22) of solid material and the cavity. Tension applied to the cable will thereby be transmitted to the strand.
The potting compound used is typically a high-strength resin. However, the term “potting compound” as used in this description means any substance which transitions from a liquid to a solid over time.
Potting is only one approach known in the art. Other common examples include “spike-and-cone” or “spike-and-barrel” designs, compression or friction fittings, composite-connections, capstan wrapping, etc. The most common approach is wrapping a length of filaments around an eye on the end of the strand and splicing a length of the strand back into itself—typically referred to as a “spliced eye.” The present invention is applicable to any method of creating a termination on the end of a synthetic filament tensile member. Although potted examples are shown in these descriptions the invention is not limited to that approach, and the reader should understand the term “strand termination” to broadly encompass all methods of attaching a device to the end of a strand.
FIG. 2 shows additional components that are added to facilitate the gathering of multiple strands into a single, load-transferring element. In the example shown, loading stud 24 has been connected to anchor 18 via threaded engagement 28. Loading stud 24 includes male thread 26 over a significant length (The threads are shown schematically but are not actually depicted for purposes of visual clarity). This threaded stud allows the completed assembly to be attached to other things to ultimately create an overall cable termination.
The use of a threaded stud is a “high-end” example. In other instances the anchor will simply be a cylinder with a load-bearing flange facing downward in the orientation of FIG. 2. The connection between the cylinder and another object could then be placing the load-bearing flange against another surface.
FIG. 3 shows the cable after an identical (in this example) strand termination 30 has been added to the end of each strand 12. The reader will observe how a length of each strand is preferably unbraided from the cable structure so that a free length exists proximate the termination. This allows each strand to be manipulated so that it may be attached to another device. A separate device or devices is used to aggregate all the individual strands and strand terminations to a unified load-transferring assembly. This unified assembly will be referred to as an “overall cable termination” in order to distinguish it from the individual “strand terminations” applied to each strand. The design of the strand terminations, the overall cable termination, and the unifying devices employed to create the overall cable termination, can take on many and various forms. The present invention is applicable to all of these forms.
As stated previously, it is ideal for each strand within a cable assembly to carry an equal percentage of the total load (other than for cables designed to distribute the load unequally). However, when a cable made of synthetic filaments is first terminated and loaded, the manufacturing tolerances will generally cause some strands to shift or “slip” relative to others—thereby altering the proportional load sharing that was intended. The present invention loads the cable in a controlled and carefully designed manner resulting in a reduction in misalignments and a more evenly distributed load among the cable's constituent strands.
Throughout this disclosure cables will be used as an example of a tensile strength member. However the invention should not be viewed as being limited to cables. The term “tensile strength member” or “tensile member” encompasses cables and sub-components of cables such as strands. The invention also encompasses non-cable structures intended to carry loads in tension.
Likewise, the term “anchor” should be viewed broadly to encompass virtually anything that can be attached to a strand or cable. The anchor would ordinarily include some features facilitating attachment—such as a hook or threads.